Abstract
Identifying traits and agents of selection involved in local adaptation is important for understanding population divergence. In southern Sweden, the moth‐pollinated orchid Platanthera bifolia occurs as a woodland and a grassland ecotype that differ in dominating pollinators. The woodland ecotype is taller (expected to influence pollinator attraction) and produces flowers with longer spurs (expected to influence efficiency of pollen transfer) compared to the grassland ecotype. We examined whether plant height and spur length affect pollination and reproductive success in a woodland population, and whether effects are non‐additive, as expected for traits influencing two multiplicative components of pollen transfer. We reduced plant height and spur length to match trait values observed in the grassland ecotype and determined the effects on pollen removal, pollen receipt, and fruit production. In addition, to examine the effects of naturally occurring variation, we quantified pollinator‐mediated selection through pollen removal and seed production in the same population. Reductions of plant height and spur length decreased pollen removal, number of flowers receiving pollen, mean pollen receipt per pollinated flower, and fruit production per plant, but no significant interaction effect was detected. The selection analysis demonstrated pollinator‐mediated selection for taller plants via female fitness. However, there was no current selection mediated by pollinators on spur length, and pollen removal was not related to plant height or spur length. The results show that, although both traits are important for pollination success and female fitness in the woodland habitat, only plant height was sufficiently variable in the study population for current pollinator‐mediated selection to be detected. More generally, the results illustrate how a combination of experimental approaches can be used to identify both traits and agents of selection.
Highlights
The two ecotypes of the moth‐pollinated orchid P. bifolia occurring in southern Sweden differ in several traits likely to influence interactions with pollinators including plant height and nectar spur length: the woodland ecotype is taller and produces nectar spurs that are longer than those produced by the grassland ecotype
We did not detect any significant interaction effects of the two traits on reproductive success, and whereas pollinators mediated significant selection for taller plants, no current pollinator‐mediated selection on spur length was detected in this population
Woodland populations grow in tall vegetation and, as expected, plant height affected both pollen removal and fruit production
Summary
Among‐population variation in interactions with pollinators may re‐ sult in divergent selection on floral traits, promoting floral diversifi‐ cation (Grant, 1949, 1994; Stebbins, 1970; Van der Niet & Johnson, 2012; Van der Niet, Peakall, & Johnson, 2014), and the formation of pollination ecotypes (Anderson, Alexandersson, & Johnson, 2010; Johnson, 1997; Van der Niet, Pirie, Shuttleworth, Johnson, & Midgley, 2014). We examined the effects of plant height (tall vs short) and spur length (long vs short) and their interaction on pollen removal and components of female fitness (the number of flowers receiving pollen, the mean number of massulae received by pollinated flowers, and the number of fruits produced) with mixed‐effect models, which included block as a random factor, using the lme package (Bates, Mächler, Bolker, & Walker, 2015) and the car package (Fox & Weisberg, 2011) to obtain type III sum of squares and p‐values in the software R. TA B L E 2 Estimates of net selection (βC; quantified in open‐pollinated control treatment, N = 119), non‐pollinator‐mediated selection (βHP; quantified as selection among plants receiving supplemental hand‐pollination, N = 58), and pollinator‐mediated selection (∆βPoll = βC−βHP) via pollen removal and female fitness in a woodland population of the orchid Platanthera bifolia on the island Öland, SE Sweden. Significant selection gradients (p < 0.05) are indicated in bold
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